1. Field of the Invention
This invention relates to microstrip transmission lines operating in the microwave and millimeter wave regions of the frequency spectrum and more particularly to a compact microstrip Y-junction circulator for use with such microstrip transmission lines.
2. Description of the Prior Art
Y-junction circulators are non-reciprocal coupling devices having three ports which provide signal transmission from one port to an adjacent port while decoupling the signal from the remaining port. Circulators of this type are used in radar system front ends as duplexers to couple the transmitter and receiver to the single radar antenna. They are also used in many other applications, such as signal generator protection circuits and transmitter injection locking circuits, for example.
At the present time, much equipment for use in the millimeter wave region of the frequency spectrum is being designed with planar circuitry using microstrip transmission lines because of the resulting substantial reduction in the size and weight of the equipment involved. In response to a need for circuit devices, such as circulators, for example, which are suitable for use with microstrip transmission lines and other planar circuitry, the applicants of the present application developed a Y-junction microstrip circulator which is shown and described in U.S. Pat. No. 4,749,966, issued June 7, 1988 to the applicants of the present application and assigned to the same assignee as the present application. The circulator described in that patent had a Y-shaped ferrite element which was mounted on the top surface of a section of microstrip dielectric substrate having an electrically conductive ground plane mounted on the bottom surface of the substrate. The ferrite element had a central portion which was shaped as a right prism having three rectangular prism faces and top and bottom prism bases shaped as equilateral triangles. Three, downwardly-sloping arm portions extended radially outwardly from the prism faces of the ferrite element. The top surfaces of the ferrite element central portion and the three arm portions had a microstrip conductor mounted thereon so that the ferrite element could be "dropped into place" as a unit on a previously prepared top surface of the substrate having three Y-oriented lengths of microstrip conductor mounted thereon. The microstrip conductor on the ferrite element was then electrically connected to the three lengths of microstrip conductor on the substrate surface so that the three lengths of microstrip conductor acted as the ports of the circulator. A permanent magnet mounted on the ground plane beneath the ferrite element provided a magnetic biasing field in the ferrite element which produced the circulator action. Although the resulting microstrip circulator was very compact and easy to fabricate, when the ferrite element was dropped into place on the substrate surface it was necessary to radially align the three arm portions of the ferrite element with the Y-oriented lengths of microstrip conductor on the substrate surface.